As accidents happen frequently, the number of amputees is considerably increasing these days. Inaptitude to move, due to injuries, often turns out to be both a pain to the patient himself and a burden to his family and the society. The only way to relieve this bad condition is rehabilitation. On the rehabilitation engineering for the amputated, Above-knee Prosthesis has been playing an important role. In general, this kind of Above-knee Prosthesis, reported by M. S. Ju et al. on JSME International Journal. (Vol.38, No.1, pp.78-86, 1995), can be divided into five main parts: socket, artificial knee joint, movement controlling unit, shank sheath, and prosthesis. Having lost their knee joints, the above-knee amputees are driven by innominate muscles to walk. The driving force is transmitted via an artificial knee joint to the stump. However, whether amputees agree to use prostheses depends mostly on the contact conditions between a stump and the socket mounted on it.
An adequate prosthesis socket should take into account the shape and sewing-up condition of a stump. For the amputees that wear prostheses, skin of the stumps may be hurt, and even worse, the subcutaneous blood circulation may be oppressed, by Normal Pressure and Shear-Stress stresses distributed on the interface owing to a close contact between the stumps and sockets. Thus, acceptability of prostheses and adaptability of amputees are badly influenced. So, if the distributions and sizes of the stresses can be measured and analyzed, and then presented for the reference of the prosthesis masters to amend the socket shapes, comfort of the prosthesis wearers will surely be improved greatly. According to the reports of M. Zhang et al. on Medical Eng. Phy., (Vol. 18, No.3, pp.207-214, 1996) a 3-D FEM model had been used to study the friction coefficient between below-knee sockets and stump skin and to measure the stresses between the contacting faces through experiments. They found that the Shear-Stress stress increased with the friction coefficient, and was more likely to hurt stump skin. As to the above-knee prostheses, M. Zhang et al. reported on IEEE Trans. on Rehabilitation Eng., (Vol. 4, No.4, pp. 337-346, 1996) that only a 2-D FEM model was made to study the friction coefficient between the contacting surfaces. They also inferred that the penetrating pain of prosthesis wearers decreased with the value of the friction coefficient, and thus suggested a reasonable coefficient should be maintained to keep a small Shear-Stress and prevent prostheses from slipping. However, no experiments were carried out to verify this inference. On clinic, most of the sockets are made according to experience. To improve the wearing comfort and enhance the design level of prostheses, therefore, it is necessary to develop a proper sensor to measure the stress and strain caused by Normal Pressure and Shear-Stress between sockets and stumps while prosthesis wearers are walking.
At present, the fluid-field micro Shear-Stress sensors are divided into two types: direct and indirect measuring. Differences between these two types: thermal Shear-Stress sensors have a simpler but stronger structure, and can be produced easily; however, it is difficult to do calibration for this type, and heating and fluid problems should also be considered carefully. It is hardly possible to equip such a system between the socket and the stump skin. The other one is the Floating element Shear-Stress sensor that has a feature of accurate dynamic calibration; however, its defects include uneasy measurement of Shear-Stress and weaker structure while using Floating element. Like the first type, it cannot be applied on the prosthesis sockets just because of no fluids existing.
As to the above-knee amputees, Shear-Stress, as well as Normal Pressure, may cause bitter or even penetrating pains on certain parts. Till now, there is no Shear-Stress sensor applied in domestic Above-knee Prosthesis researches. They are usually used to measure Shear-Stress in the fluid field by home and abroad researchers, who have paid great attention to their high sensitivity despite of their measuring range of only a few Pa's. Apparently, they are not suitable for measuring Shear-Stress between the socket and stump skin. Besides, according to the statement reported by M. Zhang et al. on IEEE Trans. on Rehabilitation Eng (Vol. 4, No. 4, pp.337-346, 1996) the maximum Normal Pressure 320K(Pa) appeared between the stump skin and the socket on it while the prosthesis wearer is standing. Therefore, while designing Shear-Stress sensors, allowable errors should be taken into consideration as a force of Normal Pressure 320K (Pa) is applied on the sensing diaphragm; i.e. expecting that almost all the signals produced by the Shear-Stress can be measured by the X-shape piezoresistors. For obtaining more accurate data, Contact-type micro-piezoresistive Shear-Stress sensors is needed which is able to endure 320 K (Pa) Normal Pressure and to measure the Shear-Stress produced between the stump skin and the socket on it. This kind of sensors should also be arranged in arrays on the surface of the stump in order to measure its Shear-Stress, distribution of pressure and other stresses, and their changes in dynamic conditions.
Meanwhile, a practical application of Above-knee Prosthesis is that: except Normal Pressure, Shear-Stress is another important factor that will affect the acceptability of prostheses and the adaptability of prosthesis wearers since the stump skin contacts so closely with the socket of a prosthesis. As the friction coefficient between the stump skin and the socket is increasing, the Shear-Stress will grow more and more, and thus the possibility to hurt the stump skin becomes greater, Therefore, a suitable friction coefficient is needed to avoid slipping of the prostheses and, at the same time, reduce the pain the prosthesis wearers may suffer. So, on the Above-knee Prosthesis rehabilitation engineering, Shear-Stress sensors, not just for vertical pressures, are urgently needed for measuring the distribution and sizes of the stresses caused by the pressures and stressed produced between the stump skin and the socket for the reference of the prosthesis masters to amend the shapes of sockets.